Disk drive employing multiple read elements to increase radial band for two-dimensional magnetic recording

ABSTRACT

A disk drive is disclosed comprising a disk comprising a plurality of tracks, and a head comprising at least three read elements including a first read element, a second read element, and a third read element. When the head is within a first radial band of the disk, data recorded on the disk is detected using the first read element and the second read element. When the head is within a second radial band of the disk different from the first radial band, data recorded on the disk is detected using the first read element and the third read element. The first read element is substantially aligned down-track with the third read element when the head is over a first radial location of the disk.

BACKGROUND

Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6 ₀-6 _(N) recorded around the circumference of each servo track. Each servo sector 6 _(i) comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6 _(i) further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

Data is typically written to data sectors within a data track by modulating the write current of a write element, for example, using a non-return to zero (NRZ) signal, thereby writing magnetic transitions onto the disk surface. A read element (e.g., a magnetoresistive (MR) element) is then used to transduce the magnetic transitions into a read signal that is demodulated by a read channel. The recording and reproduction process may be considered a communication channel, wherein communication demodulation techniques may be employed to demodulate the read signal.

When reading data from the disk, a read channel typically samples the read signal to generate read signal samples that are equalized according to a target response (e.g., a partial response). A sequence detector (e.g., a Viterbi detector) detects an estimated data sequence from the equalized samples, and errors in the estimated data sequence are corrected, for example, using a Reed-Solomon error correction code (ECC) or using a Low Density Parity Check (LDPC) code.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIG. 2A shows a disk drive according to an embodiment comprising a disk having a plurality of tracks, and a head comprising at least three read elements including a first read element, a second read element, and a third read element.

FIG. 2B is a flow diagram according to an embodiment wherein when the head is within a first radial band of the disk, data recorded on the disk is detected using the first read element and the second read element, and when the head is within a second radial band of the disk, data recorded on the disk is detected using the first read element and the third read element.

FIG. 2C shows an embodiment wherein two of the three heads are used to read data from the disk depending on the skew angle of the head.

FIGS. 3A-3C show an embodiment wherein the second and third read elements are used to read data from the disk as the head moves from the middle diameter toward the outer diameter of the disk, and then the first and third read elements are used when the skew angle of the head exceeds a threshold relative to the track pitch of the data tracks.

FIGS. 3D-3F show an embodiment wherein the first and second read elements are used to read data from the disk as the head moves from the middle diameter toward the inner diameter of the disk, and then the first and third read elements are used when the skew angle of the head exceeds a threshold relative to the track pitch of the data tracks.

FIGS. 4A and 4B show an embodiment wherein data recorded on the disk is detected using a single head and a one-dimensional demodulation algorithm at the outer and inner diameter radial bands, and data is detected using two of the heads and a two-dimensional demodulation algorithm at the inner diameter radial bands.

FIG. 5 illustrates the extent that the two-dimensional demodulation algorithm may be used to detect data recorded on the disk relative to the skew angle of the head and the track pitch of the data tracks according to an embodiment.

FIG. 6 illustrates an embodiment where the radial density of the data tracks is varied relative to the skew angle of the head and whether a one-dimensional or two-dimensional algorithm may be used to detect data recorded on the disk.

FIG. 7A shows a disk drive according to an embodiment comprising a disk having a plurality of tracks accessed using a one-dimensional or two-dimensional demodulation algorithm.

FIG. 7B is a flow diagram according to an embodiment wherein when the head is within a first radial band of the disk, data recorded on the disk is detected using a two-dimensional demodulation algorithm, and when the head is within a second radial band of the disk, data recorded on the disk is detected using a one-dimensional demodulation algorithm.

FIGS. 8A-8F show an embodiment where the first and third read elements may be fabricated with a relative radial offset (intentionally or due to manufacturing tolerances).

DETAILED DESCRIPTION

FIG. 2A shows a disk drive according to an embodiment comprising a disk 16 comprising a plurality of tracks 18, and a head 20 comprising at least three read elements including a first read element 22 ₁, a second read element 22 ₂, and a third read element 22 ₃ (FIG. 2C). The disk drive further comprises control circuitry 24 configured to execute the flow diagram of FIG. 2B, wherein when the head is within a first radial band 25A of the disk (block 26), data recorded on the disk is detected using the first read element and the second read element (block 28). When the head is within a second radial band 25B of the disk different from the first radial band (block 30), data recorded on the disk is detected using the first read element and the third read element (block 32). As shown in FIG. 2C, in one embodiment the first read element 22 ₁ is substantially aligned down-track with the third read element 22 ₃ when the head 20 is over a first radial location of the disk 16 (e.g., near the middle diameter of the disk 16). In another embodiment, the control circuitry 24 selects two of the three read elements 22 ₁-22 ₃ to detect user data recorded on the disk based on the radial location of the head (e.g., the radial band), while the unselected read element is not used to read the user data.

In the embodiment of FIG. 2A, a plurality of concentric servo tracks are defined by embedded servo sectors 34 ₀-34 _(N), wherein a plurality of concentric data tracks 18 are defined relative to the servo tracks at the same or different radial density. The control circuitry 24 processes a read signal 36 emanating from at least one of the read elements to demodulate the servo sectors and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 24 filters the PES using a suitable compensation filter to generate a control signal 38 applied to a voice coil motor (VCM) 40 which rotates an actuator arm 42 about a pivot in order to actuate the head 20 radially over the disk 16 in a direction that reduces the PES. The servo sectors 34 ₀-34 _(N) may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern.

In one embodiment, the read signal generated by at least two of the read elements are processed to detect data recorded in a target data track using a two-dimensional demodulation algorithm meaning that the inter-track interference (ITI) caused by at least one adjacent data track is compensated in order to detect the data recorded in the target data track. FIG. 3A illustrates an example of this embodiment wherein when the head 20 is at the middle diameter of the disk 16, the read signal generated by the second read element 22 ₂ and the read signal generated by the third read element 22 ₃ are processed to detect data recorded in the target data track 44B. That is, when detecting the data recorded in the target data track 44B, the ITI caused by the adjacent data track 44C is compensated by processing the read signal generated by the third read element 22 ₃. The ITI compensation may be implemented in any suitable manner, such as by subtracting the read signal generated by the third read element 22 ₃ from the read signal generated by the second read element 22 ₂ in the analog or digital domain. In another embodiment, the control circuitry 24 may employ two-dimensional digital equalization followed by a suitable two-dimensional sequence detector (e.g., a trellis type sequence detector such as a Viterbi detector). In another embodiment, the control circuitry 24 may employ two-dimensional (2D) to one-dimensional (1D) or 2D-to-1D digital equalization followed by a suitable one-dimensional sequence detector. In still another embodiment, the control circuitry 24 may process the read signal generated by the third read element 22 ₃ to detect a data sequence recorded in the adjacent data track 44C, convert the detected data sequence into ideal signal samples, and then subtract the ideal signal samples from the equalized signal samples of the read signal generated by the second read element 22 ₂. The resulting compensated signal samples may then be processed using a suitable one-dimensional sequence detector. Although in the embodiment of FIG. 3A the second read element 22 ₂ does not overlap the third read element 22 ₃ when the head 20 is near the middle diameter of the disk, in another embodiment the read elements may be fabricated so there is a small amount of overlap which may improve the ITI compensation near the middle diameter of the disk.

Referring to FIG. 3B, as the head 20 moves toward the outer diameter of the disk 16, the skew angle of the head causes the third read element 22 ₃ to overlap with the target data track 46B over which the second read element 22 ₂ is positioned. That is, part of the third read element 22 ₃ is over the target data track 46B, and part of the third read element 22 ₃ is over the adjacent data track 46C. At this skew angle, the data recorded in the target data track may still be detected by processing the read signal generated by the second and third read elements 22 ₂ and 22 ₃. As the head 20 moves further toward the outer diameter of the disk 16 as illustrated in FIG. 3C, the overlap of the second and third read elements 22 ₂ and 22 ₃ becomes excessive. That is, there is a point (e.g., up to one-quarter of a data track) where using the third read element to compensate for ITI for the second read element begins to degrade due to an excessive overlap of the read elements within the target data track. Accordingly, in one embodiment when the head skew exceeds a threshold (i.e., when the head 20 is within a second radial band 25B of the disk 16 as shown in FIG. 2A), data recorded on the disk 16 is detected using the first read element 22 ₁ and the third read element 22 ₃ as shown in FIG. 3C. That is, the first read element 22 ₁ is positioned over the target data track 48B and the third read element 22 ₃ is positioned over an adjacent data track 48A such that the third read element 22 ₃ is used to compensate for ITI when detecting the data recorded in the target data track 48B. In an alternative embodiment, the third read element 22 ₃ may be positioned over the target data track 48B and the first read element 22 ₁ used to compensate for ITI from the adjacent data track 48C. That is, the ITI may be compensated from the adjacent data track on either side of the target data track, and in one embodiment a retry operation may attempt to compensate for the ITI from each of the adjacent data tracks as well as from both of the adjacent data tracks (e.g., by buffering data during an initial revolution of the disk).

FIGS. 3D-3F illustrate a similar toggling between read elements as the head moves from the middle diameter toward the inner diameter of the disk (i.e., when the head skew is reversed). In this embodiment, the read signals from the first and second read elements 22 ₁ and 22 ₂ are processed to detect data recorded in the target data track when the head is in the first band 25A of FIG. 2A as shown in FIGS. 3D and 3E, and then when the head transitions into the third band 25C toward the inner diameter of the disk as shown in FIG. 3F, the read signals from the first and third read elements 22 ₁ and 22 ₃ are processed to detect data recorded in the target data track.

In one embodiment, the control circuitry 24 may detect data recorded in a first data track and in a second data track. Referring again to the example embodiment shown in FIG. 3A, the control circuitry 24 may process the read signals from the second read element 22 ₂ and from the third read element 22 ₃ to detect data in both data track 44B and data track 44C. The control circuitry 24 may process the read signals to detect the data from either data track using a one-dimensional or two-dimensional algorithm. For example, in one embodiment the data may be detected in both data tracks using a two-dimensional demodulation algorithm wherein the ITI from each of the adjacent data tracks is compensated. In another embodiment, the data may be detected in one of the data tracks using a one-dimensional demodulation algorithm, whereas the data recorded in the other data track may be detected using a two-dimensional demodulation algorithm.

FIGS. 4A and 4B show an embodiment wherein when the skew angle of the head 20 exceeds a threshold toward the outer diameter and inner diameter of the disk, there may be insufficient overlap of a target data track by the first and third read elements 22 ₁ and 22 ₃. Accordingly, the disk 16 may comprise an outer diameter radial band of data tracks and an inner diameter radial band of data tracks where only one of the read elements is used to detect data recorded in a target data track. In the embodiment of FIG. 4B, the read signal generated by the first read element 22 ₁ is processed to detect data recorded in a target data track when the head 20 is in the outer most diameter band of data tracks (1D-OD BAND), and the read signal generated by the third read element 22 ₃ is processed to detect data recorded in a target data track when the head 20 is in the inner most diameter band of data tracks (1D-ID BAND). However, any one of the read elements be used to read data in the 1D-OD BAND and the 1D-ID band, including the second read element

When the head is within one of the 1D-OD BAND or the 1D-ID BAND, the data recorded in a target data track is detected by processing the read signal using a one-dimensional demodulation algorithm meaning that the ITI from an adjacent data track is not compensated. When the head is within one of the 2D-OD BAND, the 2D-MD BAND, or the 2D-ID BAND, the data recorded in a target data track is detected by processing the first and second read signals using a two-dimensional demodulation algorithm. In one embodiment, the linear and/or the radial recording density of the data tracks in the 2D bands may be increased due to the improved accuracy of a two-dimensional demodulation algorithm as compared to a one-dimensional demodulation algorithm.

In an embodiment illustrated in FIG. 5, the number of data tracks spanned by the 2D bands depends on the track pitch (track width) of the data tracks (for a given configuration of the read elements). As the track pitch decreases leading to narrower data tracks, the width of the 2D bands decreases as shown in FIG. 5. Accordingly, in one embodiment the disk drive may be analyzed to determine an optimal track pitch that maximizes the capacity of each disk surface. That is, given the 2D bands enable a higher recording density than the 1D bands, there may be an optimal track pitch shown in FIG. 5 that optimizes the width of 2D and 1D bands to achieve the maximum capacity.

In one embodiment, the track pitch within the 2D bands may increase as the head nears each of the 1D bands. Referring to the example of FIG. 6, the 2D-MD BAND may comprise data tracks having the smallest track pitch, whereas the 2D-OD BAND and the 2D-ID BAND may comprise data tracks with a track pitch that increases toward the outer diameter and inner diameter of the disk, respectively. Increasing the track pitch toward the boundaries of the 2D bands may enable the 2D bands to span a greater number of data tracks, which may further optimizing the capacity of each disk surface. In the example of FIG. 6, the data tracks in the 1D bands comprise the largest track pitch since the one-dimensional demodulation algorithm does not compensate for ITI from an adjacent data track.

FIG. 7A shows a disk drive according to an embodiment comprising a disk 50 comprising a plurality of tracks, and a head 52 comprising at least two read elements (e.g., as shown in FIG. 2C). The disk drive further comprises control circuitry 54 configured to execute the flow diagram of FIG. 7B, wherein when the head is within a first radial band of the disk (block 56) such as the 2D-OD BAND, data recorded on the disk is detected by processing a first read signal generated by the first read element and by processing a second read signal generated by the second read element using a two-dimensional demodulation algorithm (block 58). When the head is within a second radial band of the disk different from the first radial band (block 60) such as the 1D-OD BAND, data recorded on the disk is detected by processing the first read signal using a one-dimensional demodulation algorithm (block 62).

Although the figures in the above embodiments show the read elements as having a width proximate the width of a data track, in an alternative embodiment one or more of the read elements may have a width that spans less than a full data track (e.g., eighty percent of a data track), and in other embodiments one or more of the read elements may have a width that spans more than a full data track. In one embodiment, the read elements of the head 20 may be fabricated with relative radial and/or down-track offsets that may be selected based on a target width for the data tracks (i.e., a target track pitch). In another embodiment, the relative radial and/or down-track offsets of the read elements may be measured by the control circuitry 24 executing a suitable calibration procedure, and then the target track pitch selected based on the measured offsets. In one embodiment, the target track pitch may also be selected based on the width of the write element, and in another embodiment the data tracks may be written in a shingled manner so that the target track pitch may be selected based on an amount of overlap of the shingled data tracks.

FIG. 2C shows an embodiment wherein the head 20 comprises three read elements fabricated with a particular configuration. However, other embodiments may employ a head 20 having more than three read elements fabricated in any suitable configuration and utilized in any suitable toggling sequence as the skew angle changes when the head 20 moves radially over the disk. FIGS. 8A-8F show an embodiment where the first and third read elements 22 ₁ and 22 ₃ may be fabricated with a relative radial offset (intentionally or due to manufacturing tolerances). In this embodiment, the first read element 22 ₁ may be substantially aligned down-track with the third read element 22 ₃ when the head 20 reaches a radial location of the disk 16 toward the OD as shown in FIG. 8B (as compared to near the ID as in the example of FIG. 2C). Also in one embodiment when the first and third read elements comprise a relative radial offset, the radial bands for employing either one-dimensional or two-dimensional demodulation may be asymmetric as compared to the symmetric example shown in FIG. 5.

In one embodiment, when data recorded on the disk is unrecoverable using a first two of the read elements, the control circuitry 24 may execute a retry operation using a different two of the read elements. Referring to the example of FIG. 3A, the control circuitry 24 may select read elements 22 ₂ and 22 ₃ when attempting to detect data in data track 44B during a first disk revolution, and if that fails, the control circuitry 24 may select read elements 22 ₁ and 22 ₂ to detect data during a retry revolution of the disk. In one embodiment, the control circuitry 24 may jog the read elements by a radial offset when selecting a different two of read elements for a retry operation. Referring to the example of FIG. 8A, if a read operation fails using read elements 22 ₁ and 22 ₂, during a retry revolution of the disk the control circuitry 24 may jog the read elements and then attempt to detect data in the same data track using a different two of read elements (e.g., read elements 22 ₁ and 22 ₃).

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein. 

What is claimed is:
 1. A disk drive comprising: a disk comprising a plurality of tracks; a head comprising at least three read elements including a first read element, a second read element, and a third read element; and control circuitry configured to: when the head is within a first radial band of the disk, detect data recorded on the disk using the first read element and the second read element; and when the head is within a second radial band of the disk different from the first radial band, detect data recorded on the disk using the first read element and the third read element, wherein when the head is over approximately a middle diameter of the disk, the first read element is substantially aligned down-track with the third read element and the second read element is offset radially from the first read element and the third read element by at least half a width of the second read element.
 2. The disk drive as recited in claim 1, wherein the control circuitry is further configured to read data from the disk using the first read element and the third read element when the head is within a third radial band of the disk, wherein: the first radial band spans a plurality of middle diameter tracks; the second radial band spans a plurality of outer diameter tracks; and the third radial band spans a plurality of inner diameter tracks.
 3. The disk drive as recited in claim 1, wherein when the head is within the first radial band the control circuitry is configured to: position the first read element at least partially over a first track; position the second read element at least partially over a second track; and process a first read signal generated by the first read element and a second read signal generated by the second read element to detect data recorded in at least one of the first track and the second track.
 4. The disk drive as recited in claim 3, wherein when the head is within the first radial band the control circuitry is configured to process the first read signal and the second read signal to detect data recorded in one of the first track and the second track using a two-dimensional demodulation algorithm.
 5. The disk drive as recited in claim 4, wherein when the head is within a third radial band of the disk outside one of the first radial band and the second radial band, the control circuitry is configured to process a read signal generated by the first read element to detect data recorded in a third track using a one-dimensional demodulation algorithm.
 6. The disk drive as recited in claim 4, wherein when the head is within the first radial band the control circuitry is configured to process the first read signal and the second read signal to detect data recorded in both the first track and the second track.
 7. The disk drive as recited in claim 1, wherein when the head is over approximately the middle diameter of the disk, the first read element is substantially aligned down-track with the third read element and the second read element is offset radially from the first read element and the third read element by approximately a full width of the second read element.
 8. A method of operating a disk drive, the method comprising: when a head is within a first radial band of a disk, detecting data recorded on the disk using a first read element and a second read element; and when the head is within a second radial band of the disk different from the first radial band, detecting data recorded on the disk using the first read element and a third read element, wherein when the head is over approximately a middle diameter of the disk, the first read element is substantially aligned down-track with the third read element and the second read element is offset radially from the first read element and the third read element by at least half a width of the second read element.
 9. The method as recited in claim 8, further comprising reading data from the disk using the first read element and the third read element when the head is within a third radial band of the disk, wherein: the first radial band spans a plurality of middle diameter tracks; the second radial band spans a plurality of outer diameter tracks; and the third radial band spans a plurality of inner diameter tracks.
 10. The method as recited in claim 8, wherein when the head is within the first radial band the method further comprises: positioning the first read element at least partially over a first track; positioning the second read element at least partially over a second track; and processing a first read signal generated by the first read element and a second read signal generated by the second read element to detect data recorded in at least one of the first track and the second track.
 11. The method as recited in claim 10, wherein when the head is within the first radial band the method further comprises processing the first read signal and the second read signal to detect data recorded in one of the first track and the second track using a two-dimensional demodulation algorithm.
 12. The method as recited in claim 11, wherein when the head is within a third radial band of the disk outside one of the first radial band and the second radial band, the method further comprises processing a read signal generated by the first read element to detect data recorded in a third track using a one-dimensional demodulation algorithm.
 13. The method as recited in claim 11, wherein when the head is within the first radial band the method further comprises processing the first read signal and the second read signal to detect data recorded in both the first track and the second track.
 14. The method as recited in claim 8, wherein when the head is over approximately the middle diameter of the disk, the first read element is substantially aligned down-track with the third read element and the second read element is offset radially from the first read element and the third read element by approximately a full width of the second read element.
 15. A disk drive comprising: a disk comprising a plurality of tracks; a head comprising at least three read elements including a first read element, a second read element, and a third read element; and control circuitry configured to: select the first read element and the second read element to detect first user data recorded on the disk when the head is at a first radial location over the disk, wherein the third read element is not used to detect the first user data; and select the second read element and the third read element to detect second user data recorded on the disk when the head is at a second radial location over the disk different from the first radial location, wherein the first read element is not used to detect the second user data.
 16. The disk drive as recited in claim 15, wherein: the first user data is the same as the second user data; the first radial location corresponds to a first read operation during a first revolution of the disk; and the second radial location corresponds to a retry read operation during a second revolution of the disk.
 17. The disk drive as recited in claim 15, wherein: the first radial location is within a first radial band of the disk; and the second radial location is within a second radial band of the disk.
 18. The disk drive as recited in claim 17, wherein when the head is within the first radial band the control circuitry is configured to: position the first read element at least partially over a first track; position the second read element at least partially over a second track; and process a first read signal generated by the first read element and a second read signal generated by the second read element to detect the first user data recorded in at least one of the first track and the second track.
 19. The disk drive as recited in claim 18, wherein when the head is within the first radial band the control circuitry is configured to process the first read signal and the second read signal to detect the first user data recorded in one of the first track and the second track using a two-dimensional demodulation algorithm.
 20. A method of operating a disk drive, the method comprising: selecting a first read element and a second read element to detect first user data recorded on a disk when a head is at a first radial location over the disk, wherein a third read element is not used to detect the first user data; and selecting the second read element and the third read element to detect second user data recorded on the disk when the head is at a second radial location over the disk different from the first radial location, wherein the first read element is not used to detect the second user data.
 21. The method as recited in claim 20, wherein: the first user data is the same as the second user data; the first radial location corresponds to a first read operation during a first revolution of the disk; and the second radial location corresponds to a retry read operation during a second revolution of the disk.
 22. The method as recited in claim 20, wherein: the first radial location is within a first radial band of the disk; and the second radial location is within a second radial band of the disk.
 23. The method as recited in claim 22, wherein when the head is within the first radial band the method further comprises: positioning the first read element at least partially over a first track; positioning the second read element at least partially over a second track; and processing a first read signal generated by the first read element and a second read signal generated by the second read element to detect the first user data recorded in at least one of the first track and the second track.
 24. The method as recited in claim 23, wherein when the head is within the first radial band the method further comprises processing the first read signal and the second read signal to detect the first user data recorded in one of the first track and the second track using a two-dimensional demodulation algorithm. 